Direct detection: We have a spin independent scattering cross section that is
constrained by several experiments such as XENON1T [1]}.
The dominant contribution is Higgs portal via interactions in the scalar potential.
However we simply assume the Higgs portal coupling \(c_{\chi _\ell \chi _\ell h}\)  for \(\chi _l-\chi _l-h_{\rm SM}\)  to be tiny
enough; \(c_{\chi _\ell \chi _\ell h} \lesssim 10^{-3}\)  [2]}, in order
to evade the direct detection via Higgs exchanges.
